Such a method is known from U.S. Pat. No. 6,744,048. This patent relates to aligning a phase plate in a TEM. In a TEM a beam of energetic electrons irradiates a thin sample. Preferably the beam is a parallel beam. The sample is so thin (typically between 25 nm and 250 nm), that most electrons pass through it unaffected, but some electrons are absorbed or scattered. In the diffraction plane (coincident with the back-focal plane of the objective lens when a parallel beam irradiates the sample) the electrons that passed the sample unhindered are all focused in one point. Scattered electrons are focused in other points in this diffraction plane.
The diffracted electrons form a phase image by destructive or constructive interference with the undiffracted electrons. A problem of said imaging method is that for low spatial frequencies the Contrast Transfer Function (CTF) shows a minimum, and thus the contrast of large structures is low.
The image can be enhanced by inserting a phase plate in the diffraction plane, the phase plate inducing a phase difference between the central, undiffracted beam and the diffracted electrons. Preferably the phase shift introduced is +π/2 or −π/2, thereby changing the sinusoidal behavior of the CTF to a cosinusoidal behavior.
The above is discussed and illustrated in, for example, “Phase contrast enhancement with phase plates in biological electron microscopy”, K. Nagayama et al., Microscopy Today, Vol 18 No 4, 2010, pp. 10-13.
The phase plate used by Nagayama, co-author of the known U.S. Pat. No. 6,744,048, is a so-called Zernike phase plate consisting of a thin carbon foil. The foil causes an electron beam passing through the foil to experience a phase shift of −π/2. For 300 keV electrons the thickness of the carbon foil should thus be approximately 24 nm. The foil shows a central hole with a diameter of 1 μm, or even 0.5 μm, through which the undiffracted beam passes. It is noted that the diameter of the central beam (the focus in which all undiffracted electrons are focused) in the diffraction plane is approximately 20 nm. The accuracy with which the phase plate must be aligned is thus better than +/−0.5 μm or even better than +/−0.25 μm.
The patent discusses the problems associated with mechanical alignment of the phase plate to the required accuracy, and proposes to use electro-magnetic alignment with coils to centre the phase plate.
Obviously the phase plate can, during the alignment, intercept the central beam. As this beam comprises most of the current with which the sample is irradiated, and the focus has a small diameter, both the current and the current density are high. The phase plate may thus be damaged due to thermal heating, and also contamination may occur, said contamination giving rise to charging when irradiated. Also beam induced oxidation may result in charging. Nowadays also TEM's become available in which the sample resides in a volume in which gasses are introduced. This is done to study micro-chemistry effects etc. The result of the higher gas pressure at the sample is that often also the pressure at, for example, the position of the phase plate is higher. This may give rise to surface oxidation of the phase plate structure, and thus to charging.
It is noted that such damaging or contamination problems may also occur when other optical elements upstream of the phase plate (upstream meaning between electron source and the phase plate) are aligned, as this may cause a shift of the image in the diffraction plane. Examples of such optical elements are deflectors, lenses and apertures positioned between electron source and the phase plate.
It is further noted that similar damage and/or contamination may occur for other phase enhancement structures, such as the knife edge used in Single Sideband Imaging, or in the so-named “tulip device” described in European Patent application EP10167258 (acting as a Single Sideband imaging device for a selected spatial frequency range), the miniature electrostatic miniature lens (see e.g. U.S. Pat. No. 5,814,815), or the phase shift element disclosed in U.S. Pat. No. 7,737,412, as all these structures must be extend to a position close to the central beam.
Accordingly, there is a need for an improved method of centering optical elements while avoiding damage or charging of the structure to enhance the Contrast Transfer Function.